FLUID POWER

Often the selection of seals is limited to seals covered by military specifications. However, there are occasions when nonstandard or proprietary seals reflecting the advancing state of the art may be approved. Thus, it is important to follow the manufacturer’s instructions when you replace seals. If the proper seal is not available, you should give careful consideration in the selection of a suitable substitute. Consult the Naval Ships’ Technical Manual, military standards, military standardization handbooks, and other applicable technical manuals if you have any doubts in selecting the proper seal. Seals are made of materials that have been carefully chosen or developed for spe- cific applications. These materials include tetrafluoroethylene (TFE), commonly called Teflon; synthetic rubber (elastomers); cork; leather; metal; and asbestos. Some of the most common materials used to make seals for fluid power systems are discussed in the following paragraphs. CORK Cork has several of the required properties, which makes it ideally suited as a sealing material in certain applications. The compressibility of cork seals makes them well suited for confined applications in which little or no spread of the material is allowed. The compressibility of cork also makes a good seal that can be cut to any desired thickness and shape to fit any surface and still provide an excellent seal. One of the undesirable characteristics of cork is its tendency to crumble. If cork is used as packing or in areas where there is a high fluid pressure and/or high flow velocity, small particles will be cast off into the system. Cork use in fluid power systems is therefore limited. It is sometimes used as gasket materials for inspection plates of hydraulic reservoirs. Cork is generally recommended for use where sustained temperatures do not exceed 275 0 F. CORK AND RUBBER Cork and rubber seals are made by combining synthetic rubber and cork. This combination has the properties of both of the two materials. This means that seals can be made with the compressibility of cork, but with a resistance to fluid comparable to the synthetic rubber on which they are based. Cork and rubber composition is sometimes used to make gaskets for applications similar to those described for cork gaskets. LEATHER Leather is a closely knit material that is generally tough, pliable, and relatively resistant to abrasion, wear, stress, and the effects of temperature changes. Because it is porous, it is able to absorb lubricating fluids. This porosity makes it necessary to impregnate leather for most uses. In general, leather must be tanned and treated in order to make it useful as a gasket material. The tanning processes are those normally used in the leather industry. Leather is generally resistant to abrasion regardless of whether the grain side or the flesh side is exposed to abrasive action. Leather remains flexible at low temperatures and can be forced with comparative ease into contact with metal flanges. When properly impregnated, it is impermeable to most liquids and some gases, and capable of withstanding the effects of temperatures ranging from –70 0 F to +220 0 F. Leather has four basic limitations. First, the size of the typical hide limits the size of the seals that can be made from leather. A second limitation is the number of seals that are acceptable. Another limitation is that under heavy mechanical pressures leather tends to extrude. Finally, many of the properties (such as impermeability, tensile strength, high- and low-temperature resistance, pliability, and compatibility with environment) depend upon the type of leather and impregnation. Leathers not tanned and impregnated for specific conditions and properties will become brittle, dry, and completely degreased by exposure to particular chemicals. Leather is never used with steam pressure of any type, nor with acid or alkali solutions. Leather may be used as packing. When molded into V’sand U’s,and cups, and other shapes, it can be applied as dynamic packing, while in its flat form it can be used as straight compression packing. METAL One of the most common metal seals used in Navy equipment is copper. Flat copper rings are sometimes used as gaskets under adjusting screws to provide a fluid seal. Molded copper rings are sometimes used as packing with speed gears operating under high pressures. Either type is 7-2

Figure 7-1.—Spiral-wouna metallic-asbestos gasket. easily bent and requires careful handling. In addition, copper becomes hard when used over long periods and when subjected to compression. Whenever a unit or component is disassembled, the copper sealing rings should be replaced. However, if new rings are not available and the part must be repaired, the old ring should be softened by annealing. (Annealing is the process of heating a metal, then cooling it, to make it more pliable and less brittle.) Metallic piston rings are used as packing in some fluid power actuating cylinders. These rings are similar in design to the piston rings in automobile engines. Metal is also used with asbestos to form spiral-wound metallic-asbestos gaskets (fig. 7-1). These gaskets are composed of interlocked plies of preformed corrugated metal and asbestos strips, called a filler. The filler may or may not be encased in a solid metal outer ring. These gaskets are used in flanged connections and for connecting the body to the bonnet in some valves, and are usually required in specific high-pressure, high-temperature applications. RUBBER The term rubber and synthetic rubbers, covers many natural each of which can be compounded into numerous varieties. The characteristics of these varieties have a wide range, as shown in table 7-1. The table shows, with the exception of a few basic similarities, that rubbers have diverse properties and limitations; therefore, specific applications require careful study before the sealing material is selected. Natural rubbers have many of the charac- teristics required in an effective seal. However, their very poor resistance to petroleum fluids and rapid aging when exposed to oxygen or ozone limit their use. Today their use has almost ceased. There are two general classes of synthetic rubber seals. One class is made entirely of a certain synthetic rubber. The term homogeneous, which means having uniform structure or composition throughout, is frequently used to describe this class of seal. The other class of seal is made by impregnating woven cotton duck or fine-weave asbestos with synthetic rubber. This class is sometimes referred to as fabricated seals. Additional information on sealing materials is provided in the Military Handbook, Gasket Materials (Nonmetalic), MIL-HDBK-212; and the Naval Ships’ Technical Manual, chapter 078. TYPES OF SEALS Fluid power seals are usually typed according to their shape or design. These types include T-seals, V-rings, O-rings, U-cups and so on. Some of the most commonly used seals are discussed in the remainder of this chapter. T-SEALS The T-seal has an elastomeric bidirectional sealing element resembling an inverted letter T. This sealing element is always paired with two special extrusion-resisting backup rings, one on each side of the T. The basic T-seal configuration is shown in figure 7-2, view A. The backup rings Figure7-2.–T-seals. 7-3 FPf07001 PERFORMED METAL STRIP METAL OUTER RING ASBESTOS FILLER FPf07002 ZERO PRESSURE BACKUP RINGS SEALING ELEMENT PRESSURE A. B. (UNPRESSURIZED) C. (PRESSURIZED)

Table 7-l.—Comparison of Physical Properties for Some Hydraulic Fluid Seal Materials Figure 7-3.—V-rings. 7-4 are single turn, bias cut, and usually made of TFE, molybdenum-disulfide-impregnated nylon, or a combination of TFE and nylon. Nylon is widely used for T-seal backup rings because it provides excellent resistance to extrusion and has low friction characteristics. The special T-ring configuration adds stability to the seal, eliminating spiraling and rolling. FPf07003

T-seals are used in applications where large clearances could occur as a result of the expansion of the thin-walled hydraulic cylinder. The T-ring is installed under radial compression and provides a positive seal at zero or low pressure. Backup rings, one on each side, ride free of T-ring flanges and the rod or cylinder wall (fig. 7-2, view B). These clearances keep seal friction to a minimum at low pressure. When pressure is applied (fig. 7-2, view C), the T-ring acts to provide positive sealing action as fluid pressure increases. One frequently used T-ring, manufactured by Greene, Tweed and Company, (called a G-Tring ®1 ), incorporates a unique, patented backup ring feature. One corner on the ID of each radius-styled backup ring on the G-Tring ® set has been rounded to mate with the inside corner of the rubber T. Figure 7-2, views B and C, shows the G-Tring ® . There is no military standard part numbering system by which T-seals can be identified. In general, each manufacturer issues proprietary part numbers to identify seals. However, it is common practice to identify T-seal sizes by the same dash numbers used for equivalent O-ring sizes (discussed later in this chapter) as defined by AS568 and MS28775 dimension standards. Typically, an O-ring groove that accepts a certain O-ring dash number will accept the same dash number T-seal. In the absence of an existing military standard for identifying T-seals, a new and simple 1 G-Tring ® is a Greene, Tweed Trademark, numbering system was created to identify T-seals required for hydraulic actuators (piston seals only) without reference to a particular manufacturer’s part number. The Navy number is composed of the letters G-T followed by a dash number of three digits and one letter, R, S, or T (for example, G-T-217T). The three digits are the appropriate O-ring size dash number according to AS568 or MS28775. The letters R, S, and T designate the number of backup rings that the groove of the T-seal is designed to accommodate: none, one, or two, respectively. V-RINGS The V-ring is one of the most frequently used dynamic seals in ship service although its identification, installation, and performance are probably most misunderstood. Properly selected and installed, V-rings can provide excellent service life; otherwise, problems associated with friction, rod and seal wear, noise, and leakage can be expected. The V-ring is the part of the packing set that does the sealing. It has a cross section resembling the letter V, (fig. 7-3) from which its name is derived. To achieve a seal, the V-ring must be installed as part of a packing set or stack, which includes one male adapter, one female adapter, and several V-rings (fig. 7-4). The male adapter is the first ring on the pressure end of the packing stack and is flat on one side and wedge-shaped on the other to contain the V of the adjacent V-ring. The female adapter, the last ring of the Figure 7-4.—Outside packed V-ring installations. 7-5 FPf07004 FEMALE ADAPTER MALE ADAPTER V-RINGS THREADED - TYPE BOLTED - TYPE

packing stack, is flat on one side and V-shaped on the other to properly support the adjacent V-ring. Proper design and installation of the female adapter has significant impact on the service life and performance of the V-rings because the female adapter bridges the clearance gap between the moving surfaces and resists extrusion. The packing set is installed in a cavity that is slightly deeper than the free stack height (the nominal overall height of a V-ring packing set, including the male and female adapters as measured before installation) and as wide as the nominal cross section of the V-rings. This cavity, called a packing gland or stuffing box, contains and supports the packing around the shaft, rod, or piston. Adjustment of the packing gland depth through the use of shims or spacers is usually necessary to obtain the correct squeeze or clearance on the packing stack for good service life. Two basic installations apply to V-ring packings. The more common is referred to as an outside packed installation, in which the packing seals against a shaft or rod, as shown in figure 7-4. The inside packed installation, is shown as a piston seal in figure 7-5. When V-ring packing is to be used in an inside packed installation, only endless ring packing should be used. Where pressures exist in both directions, as on a double-acting piston, opposing sets of packing Figure 7-5.—Inside packed V-ring installation. should always be installed so the sealing lips face away from each other as in figure 7-5. This prevents trapping pressure between the sets of packings. The female adapters in inside packed installations should always be located adjacent to a fixed or rigid part of the piston. O-RINGS An O-ring is doughnut-shaped. O-rings are usually molded from rubber compounds; how- ever, they can be molded or machined from plastic materials. The O-ring is usually fitted into a rectangular groove (usually called a gland) machined into the mechanism to be sealed. An O-ring seal consists of an O-ring mounted in the gland so that the O-ring’s cross section is compressed (squeezed) when the gland is assembled (fig. 7-6). An O-ring sealing system is often one of the first sealing systems considered when a fluid closure is designed because of the following advantages of such a system: 1. 2. 3. 4. 5. 6. 7. Simplicity Ruggedness Low cost Ease of installation Ease of maintenance No adjustment required No critical torque in clamping Figure 7-6.—O-ring installed in a gland. 7-6 FPf07005 FPf07006 ZERO PRESSURE PRESSURE VIEW A VIEW B

8. 9. 10. 11. Low distortion of structure Small space requirement Reliability Effectiveness over wide pressure and temperature ranges As stated previously, O-rings are used in both static (as gaskets) and dynamic (as packing) applications. An O-ring will almost always be the most satisfactory choice of seals in static applications if the fluids, temperatures, pressure, and geometry permit. Standard O-ring packings are not specifically designed to be used as rotary seals. When infrequent rotary motion or low peripheral velocity is involved standard O-ring packings may be used, provided consistent surface finishes over the entire gland are used and eccentricities are accurately controlled. O-rings cannot compensate for out-of-round or eccentrically rotating shafts. As rotary seals, O-rings perform satisfactorily in two application areas: 1. In low-speed applications where the surface speed of the shaft does not exceed 200 ft/min 2. In high-speed moderate-pressure appli- cations, between 50 and 800 psi The use of low-friction extrusion-resistant devices is helpful in prolonging the life and improving the performance of O-rings used as rotary seals. O-rings are often used as reciprocating seals in hydraulic and pneumatic systems. While best suited for short-stroke, relatively small diameter applications, O-rings have been used successfully in long-stroke, large diameter applications. Glands for O-rings used as reciprocating seals are usually designed according to MIL-G-5514 to provide a squeeze that varies from 8 to 10 percent minimum and 13.5 to 16 percent maximum. A squeeze of 20 percent is allowed on O-rings with a cross section of 0.070-inch or less. In some reciprocating pneumatic applications, a floating O-ring design may simultaneously reduce friction and wear by maintaining no squeeze by the gland on the O-ring. When air pressure enters the cylinder, the air pressure flattens the O-ring, causing sufficient squeeze to seal during the stroke. If the return stroke does not use pneumatic power, the O-ring returns to its round cross section, minimizing drag and wear on the return stroke. Identification As a maintenance person or supervisor working with fluid power systems, you must be able to positively identify, inspect, and install the correct size and type of O-ring to ensure the best possible service. These tasks can be difficult since part numbers cannot be put directly on the seals and because of the continual introduction of new types of seals and obsolescence of others. (Naval Ships’ Technical Manual, chapter 078, contains a table that cross-references obsolete and current O-ring specifications for ship applications.) O-rings are packaged in individually sealed envelopes. O-ring seals manufactured to govern- ment specifications are marked according to the requirements of the specific military specification and standard. The required marking for each package is as follows: 1. 2. 3. 4. 5. 6. 7. 8. 9. National stock number (NSN) Nomenclature Military part number Material specification Manufacturer’s Manufacturer’s Manufacturer’s name compound number batch number Contract number Cure date NOTE: Keep preformed packings in their original envelopes, which provide preservation, protection, identification, and cure date. When you select an O-ring for installation, carefully observe the information on the package. If you cannot positively identify an O-ring, discard it. The part number on the sealed package provides the most reliable and complete identification. 7-7

Sizes A standardized dash number system for O-ring sizes is used in many military and industrial specifications. The O-ring size is identified by a dash number rather than the actual dimensions for convenience. The basis for the dash numbers is contained in Aerospace Standard AS568. For nongasket O-rings (packing), the dash numbers are divided into groups of one hundred. Each hundred group identifies the cross section size of the O-rings within the group (table 7-2). The 900 series dash numbers contained in AS568 identify all the presently standardized straight thread tube fitting boss gaskets. With the exception of -901, the last two digits of the dash designate the tube size in 16ths of an inch. For example, the -904 size is for a 1/4-inch tube. Dimensions The critical dimensions of an O-ring are its ID, its cross sectional diameter (W), and the height and width of the residual molding flash (see fig. 7-7). Nominal dimensions have been used to describe O-ring sizes, although this practice is rapidly being replaced by the use of dash numbers. The actual inside diameter of a seal will be slightly less than the nominal ID, but the actual OD will Table 7-2.—O-Ring Dash Numbers Versus Cross Section Sizes be slightly larger than the nominal OD. For example, an AS568-429 O-ring is described in nominal dimensions as 5 inches ID by 5-1/2 inches OD by 1/4-inch W. Actual dimensions are 4.975 inches ID by 5.525 inches OD by 0.275 inches W. Specifications Material and performance requirements for O-rings are often identified in military specifications. The dimensions of these O-rings will usually be found in accompanying slash sheets (which bear the specification number and are a part of the specification) or will be identified by various drawings and standards that relate to the specification. Included among the specifications are Air Force-Navy Standards (AN), Mili- tary Standards (MS), and National Aerospace Standards (NAS). If the specification does not identify sizes, the sizes should be identified by the AS568 dash number. Usually, you can use drawings, technical manuals, and allowance parts lists (APLs) to identify replacement O-rings. (Notes 2 and 3 of table 7-1 list some of the frequently used military specifications). Cure Date A cure date is as applicable to natural or synthetic O-rings as it is to rubber hoses. This date is the basis for determining the age of O-rings. It is extremely important that the cure date be noted on all packages. Shelf Life and Expiration Date All elastomers change gradually with age; some change more rapidly than others. The shelf life for rubber products is contained in MIL-HDBK-695. Check the age of natural or synthetic rubber preformed packings before installation to determine whether they are acceptable for use. Make a positive identification, indicating the source, cure date, and expiration date. Ensure that this information is available for all packing used. Shelf life requirements do not apply once the packing is installed in a component. The expiration date is the date after which packing should not be installed. The expiration date of all packings can be determined by adding the shelf life to the cure date. 7-8

Replacement Figure 7-8 shows a typical O-ring installation. When such an installation shows signs of internal or external leakage, the component must be disassembled and the seals replaced. Sometimes components must be resealed because of the age limitations of the seals. The O-ring should also be replaced whenever a gland that has been in service is disassembled and reassembled. Often a poor O-ring installation begins when an old seal is removed. O-ring removal involves working with parts that have critical surface finishes. If hardened-steel, pointed, or sharp- edged tools are used for removal of O-rings or backup rings, scratches, abrasions, dents, and other deformities on critical sealing surfaces can result in seal failure which, in turn, can result in Figure 7-7.—Critical dimensions of an O-ring. might scratch or mar component surfaces or damage the O-ring. An O-ring tool kit is available in the supply system for O-ring in- stallation or removal. If these tools are not on hand, special tools can be made for this purpose. A few examples of tools used in the removal and installation of O-rings are illustrated in functional failure of When removing not use pointed or the equipment. or installing O-rings, do sharp-edged tools which Figure 7-8.–Typical O-ring instalation. 7-9 FPf07007 D E T A I LO FW-W D E T A I LO FA-A A A I. D. WW W W .003" MAX .005" MAX FPf07008 CYLINDER O-RING PACKING O-RING PACKING PISTON ROD PISTON HEAD

figure 7-9. These tools should be fabricated from soft metal such as brass or aluminum; however, tools made from phenolic rod, wood, or plastic may also be used. Tool surfaces must be well rounded, polished, and free of burrs. Check the tools often, especially the surfaces that come in contact with O-ring grooves and critical polished surfaces. Notice in figure 7-9, view A, how the hook-type removal tool is positioned under the O-ring and then lifted to allow the extractor tool, as well as the removal tool, to pull the O-ring from its cavity. View B shows the use of another type of extractor tool in the removal of internally installed O-rings. In view C, notice the extractor tool positioned under both O-rings at the same time. This method of manipulating the tool positions both O-rings, which allows the hook-type removal tool to extract both O-rings with minimum effort. View D shows practically the same removal as view C, except for the use of a different type of extractor tool. The removal of external O-rings is less difficult than the removal of internally installed O-rings. Views E and F show the use of a spoon-type extractor, which is positioned under the seal. After the O-ring is dislodged from its cavity, the spoon is held stationary while the piston is simultaneously rotated and withdrawn. View F is similar to view E, except that only one O-ring is installed, and a different type of extractor tool is used. The wedge-type extractor tool is inserted beneath the O-ring; the hook-type removal tool hooks the O-ring. A slight pull on the latter tool removes the O-ring from its cavity. After removing all O-rings, cleaning of the affected parts that will receive new O-rings is Figure 7-9.—O-ring tools and O-ring removal. 7-10 BACKUP RINGS CYLINDER REMOVAL TOOL REMOVAL TOOL (HOOK TYPE) EXTRACTOR TOOL (PULL TYPE) EXTRACTOR TOOL INTERNAL O-RING REMOVAL (USING PULL TYPE EXTRACTOR AND HOOK TYPE REMOVAL TOOLS) CYLINDER MOUTH PULL PULL O-RING BACKUP RINGS BACKUP RINGS CYLINDER REMOVAL TOOL (HOOK TYPE) EXTRACTOR TOOL (PUSH TYPE) EXTRACTOR TOOL REMOVAL TOOL INTERNAL O-RING REMOVAL (USING PUSH TYPE EXTRACTOR AND HOOK TYPE REMOVAL TOOLS) CYLINDER MOUTH PUSH PULL O-RING O-RING CYLIND MOUT PU BAC RIN O RING O-RING BACKUP RING (INSERTED UNDER O-RING) BACKUP RING REMOVAL TOOL (HOOK TYPE) EXTRACTOR TOOL (PULL TYPE) EXTRACTOR TOOL REMOVAL TOOL DUAL INTERNAL O-RING REMOVAL (USING PUSH TYPE EXTRACTOR AND HOOK TYPE REMOVAL TOOLS) O-RING O-RING PULL PUSH BACKUP RING REMOVAL TOOL (HOOK TYPE) EXTRACTOR TOOL (WEDGE TYPE) EXTRACTOR TOOL REMOVAL TOOL INTERNAL O-RING REMOVAL (USING WEDGE TYPE EXTRACTOR AND HOOK TYPE REMOVAL TOOLS) O-RING PULL PUSH BACKUP RING REMOVAL SPOON EXTERNAL O-RING REMOVAL (USING SPOON TYPE EXTRACTOR REMOVAL TOOLS) EXTERNAL O-RING REMOVAL (USING WEDGE TYPE EXTRACTOR AND HOOK TYPE REMOVAL TOOLS) REMOVAL TOOL (HOOK TYPE) EXTRACTOR TOOL (WEDGE TYPE) REMOVAL TOOL EXTRACTING TOOL PUSH PUSH FPf07009 AB CD EF

mandatory. Ensure that the area used for such installations is clean and free from all contamination. Remove each O-ring that is to be installed from its sealed package and inspect it for defects such as blemishes, abrasions, cuts, or punctures. Although an O-ring may appear perfect at first glance, slight surface flaws may exist. These are often capable of preventing satisfactory O-ring performance. O-rings should be rejected for flaws that will affect their performance. By rolling the ring on an inspection cone or dowel, the inner diameter surface can be checked for small cracks, particles of foreign material, and other irregularities that will cause leakage or shorten its life. The slight stretching of the ring when it is rolled inside out will help to reveal some defects not otherwise visible. A further check of each O-ring should be made by stretching it between the fingers, but care must be taken not to exceed the elastic limits of the rubber. Following these inspection practices will prove to be a maintenance economy. It is far more desirable to take care identifying and inspecting O-rings than to repeatedly overhaul components with faulty seals. After inspection and prior to installation, lubricate the O-ring, and all the surfaces that it must slide over with a light coat of the system fluid or a lubricant approved for use in the system. Consult the applicable technical instruction or Naval Ships’ Technical Manual for the correct lubricant for pneumatic systems. Assembly must be made with care so that the O-ring is properly placed in the groove and not damaged as the gland is closed. During some installations, such as on a piston, it will be necessary to stretch the O-ring. Stretch the O-ring as little and as uniformly as possible. Avoid rolling or twisting the O-ring when maneuvering it into place. Keep the position of the O-ring mold line constant. O-rings should not be left in a twisted condition after installation. If the O-ring installation requires spanning or inserting through sharp-threaded areas, ridges, slots, and edges, use protective measures, such as the O-ring entering sleeve (fig. 7-10, view A). If Figure 7-10.–O-ring installation. 7-11 FPf07010 SOFT THIN-WALL METALLIC SLEEVE INSTALLATION TOOL (PUSH TYPE) CYLINDER MOUTH SHARP EDGES, CORNERS AND THREADSO-RING RECEIVING GROOVE VIEW A PAPER ENTERING SLEEVE SHARP EDGES AND THREADS O-RING RECEIVING GROOVE INSTALLATION TOOL (PUSH TYPE) VIEW B SHARP EDGES AND CORNERS FOIL COVER THREADED AREA O-RING RECEIVING GROOVES O-RING RECEIVING GROOVES PAPER COVER EXTERNAL O-RING INSTALLATION (USING PAPER OR FOIL COVER TO AVOID O-RING DAMAGE FROM SHARP EDGES OR THREADS) VIEW C INTERNAL O-RING INSTALLATION (USING METALLIC SLEEVE TO AVOID O-RING DAMAGE FROM SHARP EDGES OR THREADS, AND PUSH TYPE INSTALLATION TOOL) INTERNAL O-RING INSTALLATION (USING FOIL OR O-RING PACKAGE ENTERING SLEEVE TO AVOID O-RING DAMAGE FROM SHARP EDGES OR THREADS, AND PUSH TYPE INSTALLATION TOOL)

the recommended O-ring entering sleeve (a soft, thin wall, metallic sleeve) is not available, paper sleeves and covers may be fabricated by using the seal package (glossy side out) or lint-free bond paper (see views B and C of fig. 7-10). After you place the O-ring in the cavity provided, gently roll the O-ring with your fingers to remove any twist that might have occurred during the installation. After installation, an O-ring should seat snugly but freely in its groove. If backup rings are installed in the groove, be certain the backup rings are installed on the correct side of the ring. BACKUP RINGS Backup rings, also referred to as retainer rings, antiextrusion devices, and nonextrusion rings, are washer-like devices that are installed on the low-pressure side of packing to prevent extrusion of the packing material. Backup rings in dynamic seals minimize erosion of the packing materials and subsequent failure of the seal. At lower pressures, backup rings will prolong the normal wear life of the packing. At higher pressures, backup rings permit greater clearances between the moving parts. Normally, backup rings are required for operating pressures over 1500 psi. Backup rings can be made of polytetra- fluoroethylene, hard rubber, leather, and other materials. The most common material currently used is tetrafluoroethylene (TFE). Backup rings are available as single-turn continuous (uncut or solid), single-turn (bias) cut, and spiral cut. See figure 7-11. Leather rings are always furnished in solid ring form (unsplit). Rings of TFE are available in all three types. Packaging and Storing Backup rings are not color-coded or otherwise marked and must be identified from the packaging labels. The dash number following the military standard number found on the package indicates the size, and usually relates directly to the dash number of the O-rings for which the backup ring is dimensionally suited. Backup rings made of TFE do not deteriorate with age and do not have shelf life limitations. TFE backup rings are provided by manufacturer either in individually sealed packages or on mandrels. If unpackaged rings are stored for a long time without the use of mandrels, a condition of overlap may develop. Overlap occurs when the ID of the backup ring becomes smaller and its ends overlap each other. To correct this overlap condition, stack TFE rings on a mandrel of the correct diameter, and clamp the rings with their coils flat and parallel. Place the rings in an oven at a maximum temperature of 177 0 C (350 0 F) for approximately 10 minutes. Do not overheat them because fumes from decomposing TFE are toxic. Remove and water-quench the rings. Store the rings at room temperature before you use them (preferably for 48 hours). Installation Care must be taken in handling and installing backup rings. Do not insert them with sharp tools. Backup rings must be inspected prior to using them for evidence of compression damage, scratches, cuts, nicks, or frayed con- ditions. If O-rings are to be replaced where backup rings are installed in the same groove, never replace the O-ring without replacing the backup rings, or vice versa. Many seals use two backup rings, one on either side of the O-ring (fig. 7-12). Two backup rings are used primarily in situations (such as a reciprocating piston seal) where alternating pressure direction can cause packing to be extruded on both sides of the gland. Figure 7-11.—Types of backup rings. 7-12 FPf07011 SPIRAL CUT SINGLE TURN BIAS CUT SINGLE TURN CONTINUOUS

Figure 7-12.—Backup ring configuration. If only one backup ring is used, place the backup ring on the low-pressure side of the packing (fig. 7-13, view A). When a backup ring is placed on the high-pressure side of the packing, the pressure against the relatively hard surface of the backup ring forces the softer packing against the low-pressure side of the gland, resulting in a rapid failure due to extrusion (fig. 7-13, view B). When dual backup rings are installed, stagger the split scarfed ends as shown in figure 7-14. When installing a spiral cut backup ring (MS28782 or MS28783), be sure to wind the ring correctly to ease installation and ensure optimum per- formance. When TFE spiral rings are being installed in internal grooves, the ring must have a right-hand Figure 7-13.–Location of a single backup ring. Figure 7-14.—Installation of cut dual backup rings. 7-13 BACKUP RINGS PRESSURE 0-RING FPf07012 FPf07013VIEW A VIEW B PRESSURE PRESSURE 0-RING 0-RING EXTRUDING BACKUP RING BACKUP RING WRONG PROPER LOCATION OF A SINGLE BACKUP RING IMPROPER LOCATION OF A SINGLE BACKUP RING RIGHT FPf07014 BACKUP RINGS SCARFED ENDS (STAGGERED) SINGLE TURN BACKUP RINGS O-RING BACKUP RINGS SCARFED ENDS (STAGGERED) SPIRAL BACKUP RINGS O-RING

Figure 7-15.–Installation of TFE back up rings (internal). 7-14 FPf07015 B. T o Prevent Overlap, Slightly Stretch the T eflon Ring before Installing it into Internal Grooves. Work the Ring into Internal Grooves by Rotating the Rod. A. Reverse the Spiral of a 5R-14 Ring (Normally RH) to a Left Hand Spiral. Note that the Leveled Ends are Opposite from Step One. CHANGING DIRECTION OF ROTATION OF SPIRAL BACKUP RINGS

spiral. Figure 7-15, view A, shows how to change the direction of the spiral. The ring is then stretched slightly, as shown in view B prior to installation into the groove. While the TFE ring is being inserted into the groove, rotate the component in a clockwise direction. This will tend to expand the ring diameter and reduce the possibility of damaging the ring. When TFE spiral rings are being installed in external grooves, the ring should have a left-hand spiral. As the ring is being inserted into the groove, rotate the component in a clockwise direction. This action will tend to contract the ring diameter and reduce the possibility of damaging the ring. In applications where a leather backup ring is called for, place the smooth-grained side of the leather next to the ring. Do not cut leather backup rings. Use a leather backup ring as one continuous ring and lubricate the ring prior to installing it, particularly the smaller sizes. If stretching is necessary for proper installation, soak the backup ring in the system fluid or in an acceptable lubricant at room temperature for at least 30 minutes. or two backup rings, depending upon the specific seal groove application and width. The Quad- Ring® seal works well in, both hydraulic and pneumatic systems. Many Quad-Ring® seal sizes have been assigned NSNs and are stocked in the Federal Supply System. Quad-Ring® seals in manu- facturer’s sizes designated as Q1 through Q88 are interchangeable with O-rings conforming to AN6227. Likewise, Quad-Ring® seals in com- mercial sizes Q101 through Q152 are inter- changeable with O-rings conforming to AN6230 in the respective dash sizes from –1 through–52. Therefore, the Quad-Ring® seal stock part number uses the AN standard O-ring designations AN6227 and AN6230 and the commercial Q dash number designation. For example, NSNs are found under such reference part numbers as AN6227Q10 and AN6230Q103. If the letter Q does not follow AN6227 or AN6230, the part number is an O-ring not a Quad-Ring® seal. If Quad-Ring® seals are not available for maintenance actions, appropriate sized O-rings can be installed and they work satisfactorily. QUAD-O-DYN® SEALS QUAD-RINGS The Quad-Ring® seal is a special configura- tion ring packing, manufactured by the Minnesota Rubber. As opposed to an O-ring, a Quad- Ring® seal has a more square cross-sectional shape with rounded corners (fig. 7-16). The Quad- Ring® seal design offers more stability than the O-ring design and practically eliminates the spiraling or twisting that is sometimes encountered with the O-ring. Quad-Rings® seals are completely inter- changeable with O-rings in the sizes offered by the manufacturer. They may be installed with one The Quad-O-Dyn®, also manufactured by Minnesota Rubber, is a special form of the Quad-Ring. The Quad-O-Dyn differs from the Quad-Ring in configuration (fig. 7-17), is harder, is subject to greater squeeze, and is made of a different material. The Quad-O-Dyn® seal also works well in O-rings glands. The Quad-O-Dyn® is used in relatively few applications. However, for difficult dynamic sealing applications, the Quad-O-Dyn® can perform better than the Quad-Ring. Quad-O- Dyn® rings are installed in submarine hydraulic systems plant accumulators. Figure 7-16.—Quad-Ring. 7-15 Figure 7-17.—Quad-O-Dyn® seal. FPf07016 FPf07017

U-CUPS AND U-PACKINGS The distinction between U-cups and U-packings results from the difference in materials used in their fabrication. The U-cup is usually made of homogeneous synthetic rubber; U-packings are usually made of leather or fabric- reinforced rubber. Special aspects of each type will be discussed separately. However, all U-cups and U-packings have cross sections resembling the letter U. Both types are balanced packings, both seal on the ID and the OD, and both are applied individually, not in stacks like V-rings. Size differences between U-cups and U-packings are usually substantial enough to prevent inter- changeability. There are a few sizes with smaller diameters and cross sections that may appear to be dimensionally equivalent but are not. Therefore, U-packings should not be substituted for U-cups (or vice versa) in any installation. U-CUPS The U-cup (fig. 7-18) has been a popular packing in the past because of installation ease and low friction. U-cups are used primarily for pressures below 1500 psi, but higher pressures are possible with the use of antiextrusion rings. For double-acting pistons, two U-cups are installed in separate grooves, back-to-back or heel-to-heel. Two U-cups are never used in the same groove. This heel-to-heel type of installation is common for single-acting (monodirectional) seals, such as U-cups and V-rings, and is necessary to prevent a pressure trap (hydraulic lock) between two packings. Installation of two U-cups with sealing lips facing each other can result in hydraulic lock and must be avoided. Leather U-Packings As a rule, leather U-packings are made with straight side walls (no flared sealing lips). See figure 7-19. The leather may be chemically treated or otherwise impregnated to improve its per- formance. Leather U-packings are available in standard sizes conforming to industrial specifica- tions. For support, the cavity of the U-packing should contain a metal pedestal ring or should be filled with a suitable material. Leather U-packings with an integral pedestal support have been installed in some submarine steering and diving ram piston seals. CUP PACKINGS Cup packings resemble a cup or deep dish with a hole in the center for mounting (fig. 7-20). Cup seals are used exclusively to seal pistons in both low- and high-pressure hydraulic and pneu- matic service. They are produced in leather, homogeneous synthetic rubber, and fabric- reinforced synthetic rubber. Although the cup packing lip flares outward, the rubbing contact is made at the lip only when the fluid pressure is low. As the fluid pressure increases, the cup heel expands outward until it contacts the cylinder wall, at which point high-pressure sealing is in effect. As the pressure loading shifts the sealing line to the cup heel, the lip is actually pulled into the cup and away from the cylinder wall. On the return stroke when the pressure is relaxed, the heel will shrink slightly, leaving only the lip in contact with the wall, avoiding unnecessary wear at the heel. For reciprocating pistons, two cups installed back-to-back in separate glands are required. FLANGE PACKINGS Flange packings are used exclusively in low- pressure, outside-packed installations, such as rod Figure 7-18.—Typical U-cup seal. Figure 7-19.—U-packing. 7-16 FPf07018 FPf07019

Figure 7-20.–Cup packing. seals. The flange made of leather, (sometimes called fabric-reinforced the hat) is rubber, or homogeneous rubber. Lip sealing occurs only on the packing ID (fig. 7-21). Flange packings are generally used only for rod seals when other packings such as V-rings or U-seals cannot be used. DIRT EXCLUSION SEALS (WIPERS AND SCRAPERS) Dirt exclusion devices are essential if a satisfactory life is to be obtained from most rod seals. The smooth finished moving rod surface, if not enclosed or protected by some sort of covering, will accumulate a coating of dust or abrasive material that will be dragged or carried into the packing assembly area on the return rod stroke. Exclusion devices called wipers or scrapers are designed to remove this coating. While the terms wiper and scraper are often used interchangeably, it is useful to reserve scraper Figure 7-21.—Typical flange packing cross section. for metal lip-type devices that remove heavily encrusted deposits of dirt or other abrasive material that would merely deflect a softer lip and be carried into the cylinder. Sometimes a rod will have both a scraper and a wiper, the former to remove heavy deposits and the latter to exclude any dust particles that remain. Whenever metallic scrapers are used with felt wipers in the same groove, the felt wiper must not be compressed nor restricted in any way that affects its function as a lubricator. A wiper installed in a seal assembly in a pneumatic application may remove too much oil from the rod, requiring some method of replacing the oil. A common remedy is to provide a periodically oiled felt ring between the wiper and the seal. Felt wipers provide lubrication to extended operating rods, thus increasing component wear life. These wipers are only used to provide lubrication to parts. Much longer life could be obtained from most seals if proper attention were given to wipers and scrapers. Often, wiper or scraper failure is not noticed when a seal packing fails. As a result, only the packing is replaced, and the same worn wiper or scraper is reinstalled to destroy another packing. Check the wiper or scraper condition upon its removal. If the wiper is worn, dirty, or embedded with metallic particles, replace it with a new one. It is usually good practice to replace the wiper every time you replace the seal and even more frequently if the wiper is readily accessible without component disassembly. If replacements are not available, wash dirty wipers that are still in good condition with suitable solvent and reinstall them. Remember that a wiper or scraper is deliberately installed as a sacrificial part to protect and preserve the sealing packing. Therefore, from a user’s standpoint, wipers and scrapers should be inspected and replaced as necessary. STORAGE OF SEALS Proper storage practices must be observed to prevent deformation and deterioration of seals. Most synthetic rubbers are not damaged by storage under ideal conditions. However, most synthetic rubbers will deteriorate when exposed to heat, light, oil, grease, fuels, solvents, thinners, moisture, strong drafts, or ozone (form of oxygen formed from an electrical discharge). Damage by exposure is magnified when rubber is under tension, compression, or stress. There are several 7-17 INSIDE FOLLOWER LEATHER CUP PACKING HEEL BACK FOLLOWER PLATE FPf07020 FPf07021ROD DIA BASE DYNAMIC SEALING AREA STATIC SEALING AREA

conditions to be avoided, which include the following: 1. Deformation as a result of improper stacking of parts and storage containers. 2. Creasing caused by force applied to corners and edges, and by squeezing between boxes and storage containers. 3. Compression and flattening, as a result of storage under heavy parts. 4. Punctures caused by staples used to attach identification. 5. Deformation and contamination due to hanging the seals from nails or pegs. Seals should be kept in their original envelopes, which provide preservation, protection, identification, and cure date. 6. Contamination by piercing the sealed envelope to store O-rings on rods, nails, or wire hanging devices. 7. Contamination by fluids leaking from parts stored above and adjacent to the seal surfaces. 8. Contamination caused by adhesive tapes applied to seal surfaces. A torn seal package should be secured with a pressure-sensitive moistureproof tape, but the tape must not contact the seal surfaces. 9. Retention of overage parts as a result of improper storage arrangement or illegible identification. Seals should be arranged so the older seals are used first. 7-18

CHAPTER 8 MEASUREMENT AND PRESSURE CONTROL DEVICES For safe and efficient operation, fluid power systems are designed to operate at a specific pressure and/or temperature, or within a pressure and/or temperature range. You have learned that the lubricating power of hydraulic fluids varies with temperature and that excessively high temperatures reduce the life of hydraulic fluids. Additionally, you have learned that the materials, dimensions, and method of fabrication of fluid power components limit the pressure and temperature at which a system operates. You have also learned of means of automatically controlling pressure in both hydraulic and pneumatic systems. Most fluid power systems are provided with pressure gauges and thermometers for measuring and indicating the pressure and/or the tempera- ture in the system. Additionally, various tempera- ture and pressure switches are used to warn of an adverse pressure or temperature condition. Some switches will even shut the system off when an adverse condition occurs. These devices will be discussed in this chapter. PRESSURE GAUGES Many pressure-measuring instruments are called gauges. However, this section will be restricted to two mechanical instruments that contain elastic elements that respond to pressures found in fluid power systems—the Bourdon-tube and bellows gauges. BOURDON TUBE GAUGES The majority of pressure gauges in use have a Bourdon-tube as a measuring element. (The gauge is named for its inventor, Eugene Bourdon, a French engineer.) The Bourdon tube is a device that senses pressure and converts the pressure to displacement. Since the Bourdon-tube displace- ment is a function of the pressure applied, it may be mechanically amplified and indicated by a pointer. Thus, the pointer position indirectly indicates pressure. The Bourdon-tube gauge is available in various tube shapes: curved or C-shaped, helical, and spiral. The size, shape, and material of the tube depend on the pressure range and the type of gauge desired. Low-pressure Bourdon tubes (pressures up to 2000 psi) are often made of phosphor bronze. High-pressure Bourdon tubes (pressures above 2000 psi) are made of stainless steel or other high-strength materials. High- pressure Bourdon tubes tend to have more circular cross sections than their lower-range counterparts, which tend to have oval cross sections. The Bourdon tube most commonly used is the C-shaped metal tube that is sealed at one end and open at the other (fig. 8-1). Figure 8-1.—Simplex Bourdon-tube pressure gauge. 8-1 RED HAND POINTER OVAL CROSS SECTION BOURDON TUBE HAIRSPRING PINION GEAR SECTOR TIP (CLOSED END) ADJUSTABLE LINK FIXED OPEN END SOCKET B. OPERATING MECHANISM A. DIAL FPf08001 100 50 0 150 200 250 400 350 300

installations. A typical reservoir for use with ground and ship installations is shown in figure 9-1. This type of reservoir is made of hot rolled steel plates and has welded seams. The ends extend below the bottom of the reservoir and serve as supports. The bottom of the reservoir is convex, and a drain plug is incorporated at the lowest point. Nonpressurized reservoirs are also used in several transport-, patrol-, and utility-type aircraft. These aircraft are not designed for violent maneuvers and, in some cases, do not fly at high altitude. Those aircraft that have nonpressurized reservoirs installed and that fly at high altitudes have the reservoirs installed within a pressurized area. (High altitude in this situation means an altitude where atmospheric pressure is inadequate to maintain sufficient flow of fluid to the hydraulic pumps.) Most nonpressurized aircraft reservoirs are constructed in a cylindrical shape (fig. 9-2). The outer housing is manufactured from a strong corrosion-resistant metal. Filter elements are normally installed internally within the reservoir to clean returning system hydraulic fluid. Some of the older aircraft have a filter bypass valve installed to allow fluid to bypass the filter if the filter becomes clogged. Reservoirs that are filled by pouring fluid directly into them have a filler (finger) strainer assembly installed in the filler well to strain out impurities as the fluid enters the reservoir. Figure 9-2.—Nonpressurized aircraft reservoir. The quantity of fluid in the reservoir is indicated by either a glass tube, a directing gauge, or a float-type rod, which is visible through a transparent dome installed on the reservoir. PRESSURIZED RESERVOIRS A pressurized reservoir is required in hydraulic systems where atmospheric pressure is insufficient to maintain a net positive suction head (NPSH) to the pump. There are two common types of pressurized reservoirs—fluid-pressurized and air-pressurized. Fluid-Pressurized Reservoir Some aircraft hydraulic systems use fluid pressure for pressurizing the reservoir. The reservoir shown in figure 9-3 is of this type. This reservoir is divided into two chambers by a floating piston. The piston is forced downward in the reservoir by a compression spring within the pressurizing cylinder and by system pressure entering the pressurizing port of the cylinder. The pressurizing port is connected directly to the pressure line. When the system is pressurized, pressure enters the pressure port, thus pressurizing the reservoir. This pressurizes the pump suction line and the reservoir return line to the same pressure. The reservoir shown in figure 9-3 has five ports—pump suction, return, pressurizing, overboard drain, and bleed. Fluid is supplied to the pump through the pump suction port. Fluid returns to the reservoir from the system through the return port. Pressure from the pump enters the pressurizing cylinder in the top of the reservoir through the pressurizing port. The overboard drain port is used to drain the reservoir while performing maintenance, and the bleed port is used as an aid when servicing the reservoir. Air-Pressurized Reservoirs Air-pressurized reservoirs, such as the one shown in figure 9-4, are currently used in many high-performance naval aircraft. The reservoir is cylindrical in shape and has a piston installed internally to separate the air and fluid chambers. Air pressure is usually provided by engine bleed air. The piston rod end protrudes through the reservoir end cap and indicates the fluid quantity. The quantity indication may be seen by inspecting the distance the piston rod protrudes from the reservoir end cap. The reservoir is provided with 9-2 PUMP SUPPLY LINE EMERGENCY RESERVOIR VENT CHECK VALVE PUMP SUPPLY LINE VENT FILTER FPf09002

Figure 9-3.—Typical fluid-pressurized reservoir. threaded openings for connecting fittings and components. Figure 9-4 shows several components installed in lines leading to and from the reservoir; however, this may not be the case in actual installation. The air relief valve, bleeder valve, and soon, may reinstalled directly on the reservoir. Because the reservoir is pressurized, it can normally be installed at any altitude and still maintain a positive flow of fluid to the pump. Figure 9-4.—Air-pressurized reservoir. Some air-pressurized reservoirs also have direct contact of fluid to gas. These reservoirs are installed in large systems and may be cylindrical or rectangular in shape. They contain an oil level indicator, a pump inlet or suction line connection, a return line, a gas pressurization and venting connection, and a drain line connection or a drain plug. These reservoirs are pressurized by air from the ship’s service air system or nitrogen banks. These reservoirs are found on board aircraft carriers and submarines. ACCUMULATORS An accumulator is a pressure storage reservoir in which hydraulic fluid is stored under pressure from an external source. The storage of fluid under pressure serves several purposes in hydraulic systems. In some hydraulic systems it is necessary to maintain the system pressure within a specific pressure range for long periods of time. It is very difficult to maintain a closed system without some leakage, either external or internal. Even a small leak can cause a decrease in pressure. By using an accumulator, leakage can be compensated for 9-3 BLEED PORT (UP WHEN INSTALLED) PRESSURIZING PORT RETURN SUCTION BLEED VALVE COMPRESSION SPRING OVERBOARD DRAIN FITTING (DOWN WHEN INSTALLED) FPf09003 AIR BLENDER VALVE FLUID RETURN AIR SUPPLY FLUID SUPPLY AIR RELIEF VALVE INDICATOR FPf09004

Figure 9-5.–Cross-section view of a piston-type accumulator with a tailrod. 9-4 BLEED VENT PISTON ROD (TAIL ROD) CYLINDER HEAD SPLIT RING FOR RETAINING CYLINDER HEAD CYLINDER (BARREL) O-RING & BACKUP RINGS (NOT VISIBLE) PISTON SEAL & BACKUP RINGS PISTON PISTON SEAL & BACKUP RINGS QUAD-RING & BACKUP RINGS HEAD SEAL CYLINDER HEAD HP AIR CONNECTION AIR CHAMBER ACCUMULATOR SUPPORT TIE ROD OIL CHAMBER TAILROD SEAL & BACKUP RINGS HOLE FOR TIE ROD HYDRAULIC OIL CONNECTION FPf09005

and the system pressure can be maintained within an acceptable range for long periods of time. Accumulators also compensate for thermal expansion and contraction of the liquid due to variations in temperature. A liquid, flowing at a high velocity in a pipe will create a backward surge when stopped suddenly by the closing of a valve. This sudden stoppage causes instantaneous pressures two to three times the operating pressure of the system. These pressures, or shocks, produce objectional noise and vibrations which can cause considerable damage to piping, fittings, and components. The incorporation of an accumulator enables such shocks and surges to be absorbed or cushioned by the entrapped gas, thereby reducing their effects. The accumulator also dampens pressure surges caused by pulsating delivery from the pump. There are times when hydraulic systems require large volumes of liquid for short periods of time. This is due to either the operation of large cylinders or the necessity of operating two or more circuits simultaneously. It is not economical to install a pump of such large capacity in the system for only intermittent usage, particularly if there is sufficient time during the working cycle for an accumulator to store up enough liquid to aid the pump during these peak demands. The energy stored in accumulators maybe also used to actuate hydraulically operated units if normal hydraulic system failure occurs. Four types of accumulators used in Navy hydraulic systems are as follows: 1. Piston type 2. Bag or bladder type 3. Direct-contact gas-to-fluid type 4. Diaphragm type PISTON-TYPE ACCUMULATORS Piston-type accumulators consist of a cylindrical body called a barrel, closures on each end called heads, and an internal piston. The piston may be fitted with a tailrod, which extends through one end of the cylinder (fig. 9-5), or it may not have a tailrod at all (fig. 9-6). In the latter case, it is referred to as a floating piston. Hydraulic fluid is pumped into one end of the cylinder and the piston is forced toward the opposite end of the cylinder against a captive Figure 9-6.—Floating piston-type accumulator. 9-5 END CAP PACKING AND BACKUP RING BARREL ASSEMBLY AIR PORT PISTON LUBRICATION PASSAGE END CAP HYDRAULIC FLUID PORT FPf09006

charge of air or an inert gas such as nitrogen. Sometimes the amount of air charge is limited to the volume within the accumulator; other installations may use separate air flasks which are piped to the air side of the accumulator. Piston accumulators may be mounted in any position. The gas portion of the accumulator may be located on either side of the piston. For example, in submarine hydraulic systems with tailrod pistons, the gas is usually on the bottom and the fluid on top; in surface ships with floating pistons, the gas is usually on the top. The orientation of the accumulator and the type of accumulator are based upon such criteria as available space, maintenance accessibility, size, need for external monitoring of the piston’s location (tailrod indication), contamination tolerance, seal life, and safety. The purpose of the piston seals is to keep the fluid and the gas separate. Usually, tailrod accumulators use two piston seals, one for the air side and one for the oil side, with the space between them vented to the atmosphere through a hole drilled the length of the tailrod. When the piston seals fail in this type of accumulator, air or oil leakage is apparent. However, seal failure in floating piston or nonvented tailrod accumulators will not be as obvious. Therefore, more frequent attention to venting or draining the air side is necessary. An indication of worn and leaking seals can be detected by the presence of significant amounts of oil in the air side. BLADDER-TYPE ACCUMULATORS Bladder- or bag-type accumulators consist of a shell or case with a flexible bladder inside the shell. See figure 9-7. The bladder is larger in diameter at the top (near the air valve) and gradually tapers to a smaller diameter at the bottom. The synthetic rubber is thinner at the top of the bladder than at the bottom. The operation of the accumulator is based on Barlow’s formula for hoop stress, which states: “The stress in a circle is directly proportional to its diameter and wall thickness.” This means that for a certain thickness, a large diameter circle will stretch faster than a small diameter circle; or for a certain diameter, a thin wall hoop will stretch faster than a thick wall hoop. Thus, the bladder will stretch around the top at its largest diameter and thinnest wall thickness, and then will gradually stretch downward and push itself outward against the walls of the shell. As a result, the bladder is capable of squeezing out all the liquid from. Figure 9-7.—Bladder-type accumulator. the accumulator. Consequently, the bladder accumulator has a very high volumetric efficiency. In other words, this type of accumulator is capable of supplying a large percentage of the stored fluid to do work. The bladder is precharged with air or inert gas to a specified pressure. Fluid is then forced into the area around the bladder, further compressing the gas in the bladder. This type of accumulator has the advantage that as long as the bladder is intact there is no exposure of fluid to the gas charge and therefore less danger of an explosion. DIRECT-CONTACT GAS-TO-FLUID ACCUMULATORS Direct-contact gas-to-fluid accumulators generally are used in very large installations where it would be very expensive to require a piston- or bladder-type accumulator. This type of accumulator consists of a fully enclosed cylinder, mounted in a vertical position, containing a liquid 9-6 AIR LIQUID FPf09007

port on the bottom and a pneumatic charging port at the top (fig. 9-8). This type of accumulator is used in some airplane elevator hydraulic systems where several thousand gallons of fluid are needed to supplement the output of the hydraulic pumps for raising the elevator platform. The direct contact between the air or gas and the hydraulic fluid tends to entrain excessive amounts of gas in the fluid. For this reason, direct contact accumulators are generally not used for pressures over 1200 psi. The use of this type of accumulator with flammable fluid is dangerous because there is a possibility of explosion if any oxygen is present in the gas, and pressure surges generate excessive heat. For this reason, safety fluids are used in this type of installation. DIAPHRAGM ACCUMULATORS The diaphragm-type accumulator is con- structed in two halves which are either screwed or bolted together. A synthetic rubber diaphragm is installed between both halves, making two chambers. Two threaded openings exist in the assembled component. The opening at the top, as shown in figure 9-9, contains a screen disc which prevents the diaphragm from extruding through the threaded opening when system pressure is depleted, thus rupturing the dia- phragm. On some designs the screen is replaced by a button-type protector fastened to the center Figure 9-8.—Direct-contact gas-to-fluid accumulator. Figure 9-9.—Diaphragm accumulator. of the diaphragm. An air valve for pressurizing the accumulator is located in the gas chamber end of the sphere, and the liquid port to the hydraulic system is located on the opposite end of the sphere. This accumulator operates in a manner similar to that of the bladder-type accumulator. FILTRATION You have learned that maintaining hydraulic fluids within allowable limits is crucial to the care and protection of hydraulic equipment. While every effort must be made to prevent contaminants from entering the system, con- taminants which do find their way into the system must be removed. Filtration devices are installed at key points in fluid power systems to remove the contaminants that enter the system along with those that are generated during normal operations. Filtration devices for hydraulic systems differ somewhat from those of pneumatic systems. Therefore, they will be discussed separately. The filtering devices used in hydraulic systems are commonly referred to as strainers and filters. Since they share a common function, the terms strainer and filter are often used interchangeably. As a general rule, devices used to remove large particles of foreign matter from hydraulic fluids are referred to as strainers, while those used to remove the smallest particles are referred to as filters. 9-7 HIGH-PRESSURE AIR LINE FOR PRECHARGING FLUID LEVEL FPf09008 TO HYDRAULIC SYSTEM SCREEN DISC DIAPHRAGM AIR CHAMBER AIR VALVE CAP FLIUD CHAMBER PRESSURE LINE FPf09009

STRAINERS Strainers are used primarily to catch only very large particles and will be found in applications where this type of protection is required. Most hydraulic systems have a strainer in the reservoir at the inlet to the suction line of the pump. A strainer is used in lieu of a filter to reduce its chance of being clogged and starving the pump. However, since this strainer is located in the reservoir, its maintenance is frequently neglected. When heavy dirt and sludge accumulate on the suction strainer, the pump soon begins to cavitate. Pump failure follows quickly. FILTERS The most common device installed in hydraulic systems to prevent foreign particles and contamination from remaining in the system are referred to as filters. They may be located in the reservoir, in the return line, in the pressure line, or in any other location in the system where the designer of the system decides they are needed to safeguard the system against impurities. Filters are classified as full flow and proportional or partial flow. In the full-flow type of filter, all the fluid that enters the unit passes through the filtering element, while in the proportional-flow type, only a portion of the fluid passes through the element. Full-Flow Filter The full-flow filter provides a positive filtering action; however, it offers resistance to flow, particularly when the element becomes dirty. Hydraulic fluid enters the filter through the inlet port in the body and flows around the filter element inside the filter bowl. Filtering takes place as the fluid passes through the filtering element and into the hollow core, leaving the dirt and impurities on the outside of the filter element. The filtered fluid then flows from the hollow core through the outlet port and into the system (fig. 9-10). Some full-flow filters are equipped with a contamination indicator (fig. 9-11). These indicators, also known as differential pressure indicators, are available in three types—gauge indicators, mechanical pop-up indicators, and electrical with mechanical pop-up indicators. As contaminating particles collect on the filter element, the differential pressure across the element increases. In some installations using Figure 9-10.—Full-flow hydraulic filter. gauges as indicators, the differential pressure must be obtained by subtracting the readings of two gauges located somewhere along the filter inlet and outlet piping. For pop-up indicators, when the increase in pressure reaches a specific value, an indicator (usually in the filter head) pops out, signifying that the filter must be cleaned or replaced. A low-temperature lockout feature is installed in most pop-up types of contamination indicators to eliminate the possibility of false indications due to cold weather because the pressure differential may be much higher with a cold fluid due to increased viscosity. Filter elements used in filters that have a contamination indicator are not normally removed or replaced until the indicator is actuated. This decreases the possibility of system contamination from outside sources due to unnecessary handling. The use of the nonbypassing type of filter eliminates the possibility of contaminated fluid bypassing the filter element and contaminating the entire system. This type of filter will minimize the necessity for flushing the entire system and lessen the possibility of failure of pumps and other components in the system. A bypass relief valve is installed in some filters. The bypass relief valve allows the fluid to bypass the filter element and pass directly through the outlet port in the event that the filter element becomes clogged. These filters may or may not be equipped with the contamination indicator. Figure 9-11 shows a full-flow bypass-type 9-8 OUT BODY FILTER ELEMENT FILTER BOWL IN FPf09010

Figure 9-11.—Full-flow bypass-type hydraulic filter (with contamination indicator). 9-9 RED DIFFERENTIAL PRESSURE INDICATOR BUTTON (RAISES 3/16 INCH WHEN DIFFERENTIAL PRESSURE EXCEEDS 70 ( 10) lb/in INDICATING A CLOGGED FILTER ELEMENT). -+ 2 MAGNETIC ACTUATOR (TYPICAL) INLET OUTLET FILTER BYPASS RELIEF VALVE OPENS AT A DIFFERENTIAL PRESSURE OF 100 ( 10) lb/in 2+- FILTER ELEMENTFILTER BOWL VIEW A NORMAL FLOW SPRING FILTER HEAD POPPET SPRING SHUTOFF DIAPHRAGM SPRING BOWL O-RING VIEW B BYPASS FLOW (CLOGGED FILTER ELEMENT) BYPASSED FLUID UNFILTERED FLUID FILTERED FLUID FPf09011

hydraulic filter with a contamination indicator. Figure 9-12 shows a full-flow bypass-type hydraulic filter without a contamination indicator. A filter bypass indicator provides a positive indication, when activated, that fluid is bypassing the filter element by flowing through the bypass relief valve. This indicator should not be confused with the pop-up differential pressure indicator previously discussed which simply monitors the pressure across the element. With the bypass indicator, a similar pop-up button is often used to signal that maintenance is needed. However, the bypass indicators further signal that, as a result of the high differential pressures across the element, an internal bypass relief valve has lifted and some of the fluid is bypassing the element. Identification of the type of installed indicator can be obtained from filter manifold drawings or related equipment manuals. Both a fluid bypass indicator and a differential pressure indicator or gauge may be installed on the same filter assembly. As with differential pressure indicators, bypass relief indicators can be activated by pressure surges, such as may develop during cold starts or rapid system pressurization. On some relief indicators, the pop-up button, or whatever signal device is used, will return to a normal position when the surge passes and pressure is reduced. Other relief indicators may continue to indicate a bypass condition until they are manually reset. Figure 9-12.—Full-flow bypass-type hydraulic filter. Before corrective action is taken based on indicator readings, the bypass condition should be verified at normal operating temperature and flow conditions by attempting to reset the indicator. Proportional-Flow Filter This type of filter operates on the venturi principle. (See glossary.) As the fluid passes through the venturi throat a drop in pressure is created at the narrowest point. See figure 9-13. A portion of the fluid flowing toward and away from the throat of the venturi flows through the passages into the body of the filter. A fluid passage connects the hollow core of the filter with the throat of the venturi. Thus, the low-pressure area at the throat of the venturi causes the fluid under pressure in the body of the filter to flow through the filter element, through the hollow core, into the low-pressure area, and then return to the system. Although only a portion of the fluid is filtered during each cycle, constant recirculation through the system will eventually cause all the fluid to pass through the filter element. Figure 9-13.Proportional-flow filter. 9-10 IN OUT BYPASS RELIEF VALVE (50 PSI) BODY FILTER ELEMENT FILTER BOWL FPf09012 VENTURI THROAT BAFFLE BODY CARTRIDGE SEDIMENT FPf09013

Filter Rating Filters are rated in several ways—absolute, mean, and nominal. The absolute filtration rating is the diameter in microns of the largest spherical particle that will pass through the filter under a certain test condition. This rating is an indication of the largest opening in the filter element. The mean filtration rating is the measurement of the average size of the openings in the filter element. The nominal filtration rating is usually interpreted to mean the size of the smallest particles of which 90 percent will be trapped in the filter at each pass through the filter. Filter Elements Filter elements generally may be divided into two classes—surface and depth. Surface filters are made of closely woven fabric or treated paper with a uniform pore size. Fluid flows through the pores of the filter material and contaminants are stopped on the filter’s surface. This type of element is designed to prevent the passage of a high percentage of solids of a specific size. Depth filters, on the other hand, are composed of layers of fabric or fibers which provide many tortuous paths for the fluid to flow through. The pores or passages must be larger than the rated size of the filter if particles are to be retained in the depth of the media rather than on the surface. Consequently, there is a statistical probability that a rather large particle may pass through a depth-type filter. Filter elements may be of the 5-micron, woven mesh, micronic, porous metal, or magnetic type. The micronic and 5-micron elements have noncleanable filter media and are disposed of when they are removed. Porous metal, woven mesh, and magnetic filter elements are usually designed to be cleaned and reused. 5-MICRON NONCLEANABLE FILTER ELEMENTS.— The most common 5-micron filter medium is composed of organic and inorganic fibers integrally bonded by epoxy resin and faced with a metallic mesh upstream and downstream for protection and added mechanical strength. Filters of this type are not to be cleaned under any circumstances and will be marked Disposable or Noncleanable. Another 5-micron filter medium uses layers of very fine stainless-steel fibers drawn into a random but controlled matrix. Filter elements Figure 9-14.—Cross-section of a stainless steel hydraulic filter element. of this material may be either cleanable or noncleanable, depending upon their construction. WOVEN WIRE-MESH FILTER ELE- MENTS.— Filters of this type are made of stainless steel and are generally rated as 15 or 25 micron (absolute). Figure 9-14 shows a magnified cross section of a woven wire-mesh filter element. This type of filter is reusable. MICRONIC HYDRAULIC FILTER ELE- MENT.— The term micronic is derived from the word micron. It could be used to describe any filter element; however, through usage, this term has become associated with a specific filter with a filtering element made of a specially treated cellulose paper (fig. 9-15). The filter shown in figure 9-10 is a typical micronic hydraulic filter. This filter is designed to remove 99 percent of all particles 10 to 20 microns in diameter or larger. Figure 9-15.—Micronic filter element. 9-11 0.0055 IN. FPf09014 FPf09015

The replaceable element is made of specially treated convolutions (wrinkles) to increase its dirt-holding capacity. The element is noncleanable and should be replaced with a new filter element during maintenance inspections. MAGNETIC FILTERS.— Some hydraulic systems have magnetic filters installed at strategic points. Filters of this type are designed primarily to trap any ferrous particles that may be in the system. PNEUMATIC GASES Clean, dry gas is required for the efficient operation of pneumatic systems. Due to the normal conditions of the atmosphere, free air seldom satisfies these requirements adequately. The atmosphere contains both dust and impurities in various amounts and a substantial amount of moisture in vapor form. Solids, such as dust, rust, or pipe scale in pneumatic systems, may lead to excessive wear and failure of components and, in some cases, may prevent the pneumatic devices from operating. Moisture is also very harmful to the system. It washes lubrication from moving parts, thereby aiding corrosion and causing excessive wear of components. Moisture will also settle in low spots in the system and freeze during cold weather, causing a stoppage of the system or ruptured lines. An ideal filter would remove all dirt and moisture from a pneumatic system without causing a pressure drop in the process. Obviously, such a condition can only be approached; it cannot be attained. Removal of Solids The removal of solids from the gas of pneumatic systems is generally done by screening (filtering), centrifugal force, or a combination of the two. In some cases, the removal of moisture is done in conjunction with the removal of solids. Some types of air filters are similar in design and operation to the hydraulic filters discussed earlier. Some materials used in the construction of elements for air filters are woven screen wire, steel wool, fiber glass, and felt fabrics. Elements made of these materials are often used in the unit that filters the air as it enters the compressor. Porous metal and ceramic elements are commonly used in filters that are installed in the compressed air supply lines. These filters also use a controlled air path to provide some filtration. Internal design causes the air to flow in a circular path within the bowl (fig. 9-16). Heavy particles and water droplets are thrown out of the airstream and drop to the bottom of the bowl. The air then flows through the filter element, which filters out most of the smaller particles. This type of filter is designed with a drain valve at the bottom of the bowl. When the valve is opened with air pressure in the system, the accumulation of solids and water will be blown out of the bowl. An air filter that uses moving mechanical devices as an element is illustrated in figure 9-17. As compressed air passes through the filter the force revolves a number of multi-blade rotors at high speed. Moisture and dirt are caught on the blades of the rotors. The whirling blades hurl the impurities by centrifugal force to the outer rims of the rotors and to the inner walls of the filter housing. Here, contaminating matter is out of the airstream and falls to the bottom of the bowl where it must be drained at periodic intervals. Removal of Moisture The removal of moisture from compressed air is important for a compressed air system. If air at atmospheric pressure, even at a very low relative humidity, is compressed to 3000 or 4500 psi, it becomes saturated with water vapor. Some moisture is removed by the intercoolers and aftercoolers (see glossary). Also, air flasks, receivers, and banks are provided with low point drains to allow periodic draining of any collected moisture. However, many uses of air require air with an even smaller moisture content than can be obtained through these methods. Moisture in Figure 9-16.—Air filter. 9-12 OUTLET INLET FPf09016

Figure 9-17.—Air filter using rotating blades as element. air lines can create problems which are potentially hazardous, such as the freezing of valves and controls. This can occur, for example, if very high pressure air is throttled to a very low pressure at a high flow rate. The venturi effect of the throttled air produces very low temperatures which will cause any moisture in the air to freeze into ice. This makes the valve (especially an automatic valve) either very difficult or impossible to operate. Also, droplets of water can cause serious water hammer in an air system which has high pressure and a high flow rate and can cause corrosion, rust, and dilution of lubricants within the system. For these reasons, air driers (dehydrator, air purifier, and desiccator are all terms used by different manufacturers to identify these components) are used to dry the compressed air. Some water removal devices are similar in design and operation to the filters shown in figures 9-16 and 9-17. Two basic types of air dehydrators are the refrigerated-type and the desiccant-type. REFRIGERATED-TYPE DEHYDRATORS.– In refrigerated-type dehydrators, compressed air is passed over a set of refrigerated cooling coils. Oil and moisture vapors condense from the air and can be collected and removed via a low point drain. DESICCANT-TYPE DEHYDRATORS.– A desiccant is a chemical substance with a high capacity to absorb water or moisture. It also has the capacity to give off that moisture so that the desiccant can be reused. Some compressed air system dehydrators use a pair of desiccant towers (flasks full of desiccant). One is kept in service dehydrating the compressed air, while the other one is being reactivated. A desiccant tower is normally reactivated by passing dry, heated air through it in the direction opposite the normal dehydration airflow. Another type of chemical drier is shown in figure 9-18. This unit consists of the housing, a cartridge containing a chemical agent, a filter (sintered bronze), and a spring. Various types of absorbent chemicals are used by the different manufacturers in the construction of the cartridges. To ensure proper filtering, the air must pass through the drier in the proper direction. The correct direction of flow is indicated by an arrow and the word FLOW printed on the side of the cartridge. Figure 9-18.–Chemical drier. 9-13 OUT IN DRAINFPf09017 CARTRIDGE HOUSING SPRING OUTLET FILTER FILTERDESICCANT FLOW FLOW DRIER CARTRIDGE FILTERCARTRIDGE CARTRIDGE AND HOUSING CARTRIDGEFPf09018 INLET

(no extractable text on this page)

CHAPTER 10 ACTUATORS One of the outstanding features of fluid power systems is that force, generated by the power supply, controlled and directed by suitable valving, and transported by lines, can be con- verted with ease to almost any kind of mechanical motion desired at the very place it is needed. Either linear (straight line) or rotary motion can be obtained by using a suitable actuating device. An actuator is a device that converts fluid power into mechanical force and motion. Cylinders, motors, and turbines are the most common types of actuating devices used in fluid power systems. This chapter describes various types of actuating cylinders and their applications, different types of fluid motors, and turbines used in fluid power systems. CYLINDERS An actuating cylinder is a device that converts fluid power to linear, or straight line, force and motion. Since linear motion is a back-and-forth motion along a straight line, this type of actuator is sometimes referred to as a reciprocating, or linear, motor. The cylinder consists of a ram or piston operating within a cylindrical bore. Actuat- ing cylinders may be installed so that the cylinder is anchored to a stationary structure and the ram or piston is attached to the mechanism to be operated, or the piston or ram may be anchored to the stationary structure and the cylinder attached to the mechanism to be operated. Actuating cylinders for pneumatic and hydraulic systems are similar in design and operation. Some of the variations of ram- and piston-type actuating cylinders are described in the following paragraphs. RAM-TYPE CYLINDERS The terms ram and piston are often used interchangeably. However, a ram-type cylinder is usually considered one in which the cross-sectional area of the piston rod is more than one-half the cross-sectional area of the movable element. In most actuating cylinders of this type, the rod and the movable element have equal areas. This type of movable element is frequently referred to as a plunger. The ram-type actuator is used primarily to push rather than to pull. Some applications require simply a flat surface on the external part of the ram for pushing or lifting the unit to be operated. Other applications require some mechanical means of attachment, such as a clevis or eyebolt. The design of ram-type cylinders varies in many other respects to satisfy the requirements of different applications. Single-Acting Ram The single-acting ram (fig. 10-1) applies force in only one direction. The fluid that is directed into the cylinder displaces the ram and forces it outward, lifting the object placed on it. Since there Figure 10-1.—Single-acting ram-type actuating cylinder. 10-1 FPf10001 PACKED GLAND FLUID SUPPLY RAM GLAND NUT CYLINDER HOUSING